Hello, Province: Cyrodiil, I've been interested in helping with this project for some time, but haven't had the time recently to complete my showcase. However, I had some thoughts on a questline for the Dark Brotherhood faction of Cyrodiil I wanted to share.

I'm not aware of what your thoughts are on connecting events between Province: Cyrodiil and those of TES IV, but as a word of warning: this questline would be somewhat of a setup to the state of the Brotherhood in TES IV, but not necessarily the quests in that game. In general, the Brotherhood is a very different, and much larger, organization from what you see in Oblivion. This questline is the beginning of the end for the Brotherhood, so to speak, as the player would see it become what it is in Oblivion, that ultimately becomes the sad remnant you see in Skyrim.

The Quest

This quest would require the following prerequisite events to have been completed:

1. The Nerevarine has defeated the Dark Brotherhood beneath Mournhold.

2. The Nerevarine has completed the first few events of P:C's main quest (to indicate some time has passed since the player entered Cyrodiil).

After these events have been completed, once the player character has gone to sleep, they are awoken by none other than Lucian Lachance, approaching you at Vicente Valtieri's request.
He speaks to the Nerevarine: "You've proven yourself most efficient, %playername%, few escape the children of our sweet mother, fewer still manage to silence an entire sanctuary. Word of your actions in Mournhold have spread so far as to reach the ears of our family's patriarch... And he is quite interested in one with your... abilities. Should you be interested in this opportunity, enter the abandoned house on the east end of Cheydinhal, near the Chapel of Arkay. He waits for you there."

Upon entering the house, you are met by Valtieri and two personal guards, one of which being Ocheeva. Vicente speaks with the Nerevarine concerning the current state of the Brotherhood, how he and his "clandestine family" believe the Brotherhood has become too far-spread, too political, and their true purpose has been muddied by profit and those who seek to earn it, unlike true children of the Night Mother. They propose you join their family, to assist them in restoring the Brotherhood to what it once was.

Should you agree, you'll begin receiving your orders via "dead drops" across Cyrodiil, the written locations of which will be found hidden in different locations within the abandoned home in Cheydinhal, and your rewards placed within a chest when you return (complete a quest, return to the abandoned home for your reward, while there, search around for the hidden dead drop location, follow the written instructions to the dead drop for your quest details). While this may seem complex, it is reasonably so, the utmost secrecy would of course be taken to keep the rest of the Brotherhood from learning of and interfering in your actions.

Your first quests will deal with you assassinating single members of the Brotherhood on their missions, thinning the herd, as it were. They attempt assassinating someone, and you assassinate them on their mission, with certain bonus rewards being offered for killing them in specific ways, leaving something on their corpse, etc.

Later missions will involve going after multiple members at once, as you begin foiling the more sensitive Brotherhood contracts that require multiple members, in addition to some sanctuaries becoming wary and sending multiple assassins instead of one. These assassins will be better than the last and more suspicious, so players will often deal with having to separate the members via traps, doors, spells, and anything they can use to take the assassins on one at a time. Each mission wil require the player to not only kill each assassin, but recover something from their corpses to bring to Ocheeva. Missions will be given by Ocheeva, who will be masquerading as a pilgrim and found at a different Chapel, wayshrine, or holy site each time you seek her out.

Before the final missions, the players will be contacted once again by Lachance: "Our patriarch wishes to speak with you once again, Brother. Our plans have reached their final stages, and for what comes next, Sithis will recognize and reward us. Enter the basement of the abandoned home, and attempt to open the black door. You will be asked a question. Answer thusly: "Sanguine, my Brother" You will gain entrance to the Sanctuary."
Once within, the player will speak with Valtieri, who will explain the sensitivity of future matters. He will reveal to you that the items you've retrieved from Ocheeva's missions, while seeming like junk, offered clues to the location of the different sanctuaries within Cyrodiil, and Lachance has managed to follow these clues and decipher the locations. You will be tasked with "the purification" of all sanctuaries within Cyrodiil.
This is where the real questline begins. Each sanctuary will be very different from the last, hidden in different locations within or near Cyrodiil's cities, and will require different tactics to purify each one. After the second sanctuary is wiped out, the others will catch on to what is happening and their defenses will be raised (explains the rising difficulty in the quests). (Perhaps these purifications will be spread out to happen between Ocheeva's quests?)

Once all the sanctuaries, save for Cheydinhal, have been wiped out, the player will speak with Valtieri, who will give them their final reward, offer some final words of thanks, and inform them that should they be up to the challenge, Ocheeva has some final tasks for them to complete.

Epilogue/Final Tasks:

Speaking with Ocheeva, the player will be informed that while the majority of the impure Brotherhood are dead or of no danger, some members of the Black Hand managed to escape. There would be three or four epilogue missions tracking down and dealing with these Black Hand members while Lachance handles the others. These missions would be rather long, with detective-level tracking down of the members (finding hidden items in specific locations, basically, what Lachance had to do to find the Sanctuaries). Between these missions, one would return to Ocheeva for a great reward and witness the Sanctuary becoming over time what it was in Oblivion. These would culminate in legitimate fights with the Black Hand members, not simple assassinations. This would culminate in the final mission, where Ocheeva informs you that Lachance has disappeared, and you are to find him. You follow the leads to the location of the final Black Hand member Lachance was to kill and find the member's body, brutally mutilated, and a letter from Lachance explaining he had left to find the killer and would hopefully return soon with a new recruit, as not just anyone can kill a Black Hand member. You return to the Sanctuary to find Ocheeva, who gives you a reward and informs you Lachance has returned with a new recruit, and gives you a bonus for the trouble of searching for a man who just returned. Should the player seek Lachance, they may have some final words with him, and speak with this new recruit, a Breton named Mathieu Bellamont.

Some Interaction with the Morag Tong:

Obviously, a Morag Tong member joining the DB would normally cause some major problems for those involved. However, should the player character speak with any of the higher ranking Morag Tong members on Vvardenfell, they will allow you to (or suggest that you, depending on your rank in the guild) assist Valtieri, as they imagine his plans will only serve to weaken the Brotherhood.

welcome to our forums.
As for the idea at hand, I'm afraid I'm not very familiar with the Dark Brotherhood in Oblivion or Skyrim and thus cannot tell much about the quality of your idea. That you would be mainly killing fellow brothership members seems like an interesting twist, though. I don't think we need to serve an explanation for the state of the DB in Oblivion, it is probably mainly a thing of different atmosphere and presentation. That's my take on the matter, at least.

Well, the Brotherhood seem to be a self destructive group largely due to their conflicting ideals and lack of many established rules or customs, unlike the Morag Tong. Where the Tong are more anuic, having rules and methods for going about any situation, the Brotherhood are padomaic, having shed that sense of order when breaking away from the Tong. I think it's important for this chaos to be evident in Dark Brotherhood questlines, as it was in Oblivion and Skyrim.